1. Field of the Invention
The present invention relates to measurement of blood flow in a limb of a living being, and particularly to an improved apparatus and method adapted to measure total venous blood flow in a limb with a flowmeter responsive only to pulsatile flow.
2. Description of the Prior Art
The circulatory system in humans and other vertebrates includes an arterial network which distributes blood from the heart to the extremities and a venous network which conveys the blood back to the heart. The arterial and venous networks are connected through capillaries which distribute the blood to the tissues.
The normal flow of blood through the arteries can be divided into variable and steady components. The variable component usually is called pulsatile flow, modulated flow, or AC flow (by analogy to electric current). The steady component is called non-modulated, residual, or DC flow. The pulsatile arterial flow component can be further subdivided into forward flow (outward from the heart to the extremities) which occurs during the systolic period of the heart cycle, and reverse (retrograde) flow, which may occur during the diastolic period.
Due to the elasticity of the arterial blood vessels, there is typically a small, fairly steady residual forward flow during the post-diastolic period. This constant, or non-modulated, residual flow can comprise a significant percentage of the total net forward flow during each heart cycle. The total net forward arterial flow is the algebraic sum of the pulsatile systolic forward flow, the pulsatile diastolic flow, and the steady residual forward flow.
Since the capillaries provide a filtering barrier between the arteries and the veins, the venous return flow is essentially steady. Under equilibrium conditions, the venous outflow through a limb is equal to the arterial inflow.
Researchers and physicians investigating the condition of patients suffering from heart or circulatory impairment are often interested in determining the rate of blood flow perfusing a limb. Non-invasive blood flowmeters of the type described in U.S. Pat. No. 3,659,591; No. 3,759,247; and No. 3,809,070 (assigned to the assignee of this invention and incorporated herein by reference) have many advantages stemming from the use of sensing electrodes placed on the skin instead of subcutaneously. However, offset voltages are developed between the electrode surfaces and the skin layers. These offset voltages tend to drift with time and mask the signal contribution of steady flow components. Consequently, such instruments are designed to be not responsive to steady flow, only to variations in flow rate.
When such an instrument is used to measure normal blood flow in a limb, therefore, it will measure only the pulsatile components, both forward and retrograde, of the arterial flow. It will not respond either to the steady residual arterial flow or to the steady venous outflow.